In the design of a transmission modulation apparatus, generally, a trade-off relationship holds between efficiency and linearity. However, recently, technology to enable compatibility between efficiency and linearity has been proposed, including polar modulation transmission technology.
FIG. 1 illustrates a configuration example of a polar modulation transmitting apparatus. Polar modulation transmitting apparatus 10 is provided with amplitude/phase data forming section 11, phase modulator 12, high frequency power amplifier (also called “power amplifier”) 14 and power supply voltage forming apparatus 13 that forms the power supply voltage VCC of high frequency power amplifier 14.
Amplitude/phase data forming section 11 forms a baseband amplitude signal S1 and baseband phase signal S2 from a transmission signal that is received as input. Here, when the in-phase component of the transmission signal is represented by “I” and the quadrature component of the transmission signal is represented by “Q,” the baseband amplitude signal S1 is represented by √(I2+Q2). The baseband phase signal S2 is the phase component of the transmission signal (e.g., the angle formed between the modulation symbol and the I-axis).
Phase modulator 12 forms a high frequency phase modulation signal S3 by modulating a carrier frequency signal by the baseband phase signal S2, and outputs this to the signal input terminal of high frequency power amplifier 14.
Based on the baseband amplitude signal S1, power supply voltage forming apparatus 13 forms a power supply voltage VCC to supply to the power supply terminal of high frequency power amplifier 14.
By this means, in high frequency power amplifier 14, the signal multiplying the power supply voltage value VCC and the high frequency phase modulation signal S3 is amplified by the gain in high frequency power amplifier 14, and the resulting transmission output signal is outputted. This transmission output signal is transmitted from an antenna (not shown).
With this polar modulation transmission technology, the high frequency phase modulation signal S3 that is received as input in high frequency power amplifier 14 is a constant envelope signal having no fluctuation component in the amplitude direction, so that it is possible to use an efficient non-linear amplifier as high frequency power amplifier 14.
By the way, this polar modulation transmitting apparatus 10 is required to establish a proportional relationship between the power supply voltage value VCC formed based on the baseband amplitude signal S1 and the output voltage of high frequency power amplifier 14 (generally calculated by converting the transmission output signal in the figure into a voltage subject to 50 Ωresistance).
Here, a HBT (Hetero-junction Bipolar Transistor) device, which allows higher gain than a FET (Field Effect Transistor) device and which can be miniaturized easily, is often used as an element that is used in high frequency power amplifier 14. However, a HBT device has a specific parameter called “offset voltage” between the power supply voltage value and output voltage.
FIG. 2 illustrates the relationship between the power supply voltage value VCC and output voltage, in the case of forming high frequency power amplifier 14 using a HBT device. In this figure, the solid line represents the relationship between the power supply voltage VCC and output voltage in the case of using a HBT device, and, although the power supply voltage VCC and output voltage change linearly, the line does not pass the origin and is therefore understood not to represent a proportional relationship. The offset voltage is the power supply voltage value at the time when the output rises, and FIG. 2 shows the relationship between the power supply voltage VCC and output voltage with linear approximation, and defines the intersection point of this line and the x axis as the offset voltage.
Up till now, it is proposed that, to control the output power of high frequency power amplifier 14 (i.e. the power of transmission output signals) in polar modulation transmitting apparatus 10, correction is performed to establish a proportional relationship between the power supply voltage VCC and output power by adjusting the level of the baseband amplitude signal S1 and adding the offset voltage shown in FIG. 2 to the level-adjusted baseband amplitude signal in power supply voltage forming apparatus 13 (e.g. see Patent Document 1). By this means, it is possible to prevent distortion due to offset from being produced.
This configuration will be briefly explained using FIG. 3. In power supply voltage forming apparatus 13 in FIG. 3, level adjusting section 21 receives as input a baseband amplitude signal S1. Then, level adjusting section 21 adjusts the level of the baseband amplitude signal S1 according to, for example, scaling coefficients from a transmission power control section (not shown), and outputs the level-adjusted baseband amplitude signal to offset adding section 23. Offset adding section 23 forms the power supply voltage VCC of high frequency power amplifier 14 by adding an offset voltage generated in offset voltage generating section 22 to the level-adjusted baseband amplitude signal, and supplies the formed voltage to the power supply terminal of high frequency power amplifier 14.    Patent Document 1: U.S. Pat. No. 6,998,919, specification